Chondrules, such as the one shown here, are millimeter-scale, previously molten droplets that make up more than half the volume of most meteorites on Earth. That ubiquity, along with isotopic dating, suggests they formed during the roughly five million years when the solar nebula was coalescing into solids (see the article by Robin Canup, Physics Today, April 2004, page 56). Unfortunately, there’s no consensus as to how. A leading theory contends that chondrules originated as rapidly melted dust clumps, probably heated by shock waves in the nebula, that recrystallized and became part of accreting planetesimals and planetary embryos. MIT’s Brandon Johnson and his colleagues now argue for an alternative model: that chondrules formed out of the melt produced when those embryos, some as massive as the Moon, collided. In their computer simulations, the researchers took into account a process known as impact jetting, in which shock-melted and vaporized material at the point of collision gets squirted into a high-speed plume. For impact speeds above 2.5 km/s, the only material that escapes is a jetted mixture of melt (nascent chondrules) and lightly shocked solids in which the chondrules eventually become embedded. Jets eject a relatively small amount of melt, typically 1% of the impactor’s mass. Nevertheless, the researchers estimate that enough impact-jetting collisions occurred in the early solar system to have produced the chondrules we see today in meteorites. (B. C. Johnson et al., Nature 517, 339, 2015.)
Figure courtesy of David A. Krieg.
